WASHING, DRYING AND GLAZING OF PAPER PRINTS 607. Methods of Washing Prints. In the case of prints on paper, the part played by washing differs in no way from that which has been studied previously (§ 415) in the case of photo graphs on impermeable (glass) and practically impermeable (film) supports. The mechanism of washing, however, is slightly different, the interchanges being considerably slower in the interior of the paper (fibres and sizing) than in the gelatine layer. Washing may be controlled by the same methods already indicated in the case of plates and For the reasons already given (§ 421) so-called hypo-eliminators should never be used in any circumstances.
When a gelatine coating on an impermeable support is being washed, the concentration of the salt solution in the coating is reduced by a constant fraction of its former concentration after each change of water (§ 416), or at the end of equal periods of washing in running water (§ 417), the soluble salt being expelled suffi ciently quickly for it to be necessary for the water to be renewed very rapidly. On the other hand, the diffusion of sodium thiosulphate and of soluble silver complex thiosulphate from the paper and its baryta coating is much slower, and is lessened as the quantity remaining to be eliminated becomes smaller. Under such conditions there is no longer any advantage in employing such a rapid renewal of water for papers as in the case of plates and films, the rate of washing being governed by the rate of elimination of salts from the support. 2 In order to bring about practically complete elimination of removable salts, it is necessary that thin papers should be washed for 30 minutes and very thick papers (cardette, post cards) for go minutes, using running water or water which is renewed every five minutes. In the former case it is sufficient to employ moderately fast running water, so long as the prints are moved about fairly frequently to prevent the massing of them together which prevents the penetration and renewal of water. Clean water must have
constant access to both sides of the prints.
When using running water for washing, it is preferable to make use of an arrangement which keeps the prints moving inside the receptacle. When washing in changes of water, it is essential that the prints be removed one by one from the dish of contaminated water and placed in the dish of fresh water. The removal of prints in a mass from one dish to another, or emptying and refilling a dish on the bottom of which the prints are heaped, is absolutely ineffective.
An excellent method of hastening the elimin ation of salts retained by photographic papers consists in pressing-out the liquid with which they are saturated from time to time (Lumiere and Seyewetz, 1902). For example, at each change of water the prints are gathered in a heap on a flat and rigid support, and the water pressed out with the hand, or, better still, by pressure with a roller (a wooden rolling pin or a thick round glass bottle can be used for the purpose). The prints, having been squeezed in this way, are placed in a small quantity of clean water from which they are taken one by one and transferred to the washing tank, which, in the meantime, has been emptied and refilled. When only a very small number of prints have to be washed, they may be pressed between blotting paper rather than treated as above.
6o8. Apparatus for Washing Prints. Print washers are often less efficacious than plate washers, the prints usually tending to heap themselves up on the bottom of the tank or on the perforated false bottom. The use of floats provided with clips for holding the prints has often been suggested, but the use of such devices is hardly possible except when washing a very small number of prints, and in that case it is just as practicable to carry out the washing in several changes of water, transferring the prints, one by one, from one tank or dish to another.